The two major determinants of long-term survival after allongeneic marrow transplantation for acute lymphocytic leukemia (ALL) are the occurrence of cytomegalovirus (CMV) infection and recurrence (relapse) of leukemia. An ongoing study has investigated the safety and toxicity of human leukocyte interferon given after transplant for ALL in remission. Interferon has been shown to be safe when given as early as 2-3 weeks after transplant though, as expected, the circulating neutrophil counts of interferon recipients are lower than those of non-recipients. Between 20 and 80 days after transplant the natural killer (NK) actvity of cells taken from interferon recipients is higher than that of non-recipients and no further augmentation of NK activity occurs after exposure of these cells to interferon in vitro. Although the clinical effects of interferon given prohylactically can only be analyzed preliminarily, it appears that a lower relapse rate may be occurring among interferon recipients but no significant change in CMV infection. It is proposed to complete the present study of the prophylactic use of interferon and to extend it to patients with other types of leukemia; these patients will receive interferon beginning one week befere transplant and will also have the sensitivity of their leukemia cells to interferon examined in vitro before transplant. These latter data will later be compared to the duration of remission after transplant. Finally, the use of interferons alone and in combination with other antiviral agents will be examined in vitro for toxicity to marrow progenitor cells.